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The impact of amino acids on growth performance and major volatile compound formation by industrial wine yeast

McKinnon, Alexander (2013-12)

Thesis (MSc)--Stellenbosch University, 2013.

ENGLISH ABSTRACT: Nitrogen composition of grape must is highly variable and impacts on the health of the
fermenting yeast population as well as the formation of aroma and flavour compounds in wine.
Insufficient yeast assimilable nitrogen (YAN), mostly consisting of amino acids and ammonium,
can lead to stuck or sluggish fermentations as well as the formation of undesirable compounds
such as H2S. Furthermore, it is well established that the total concentration of YAN and the
specific amino acid composition have a significant impact on the final aroma and flavour of
wines. However, the impact of individual amino acids and of specific amino acid compositions
on fermentation kinetics and on the production of aroma and flavour impact compounds under
winemaking conditions is not well understood.
The first goal of this study was to evaluate the effect of single amino acids on growth
kinetics and major volatile production of two industrial wine yeast strains under conditions
resembling wine fermentations. To facilitate these fermentation conditions while also allowing
for easy reproducibility and manipulation of the initial components, a synthetic grape media was
utilized. Biomass formation, exponential growth rate, lag phase, and fermentation duration were
utilized to evaluate the efficiency of single amino acids.
The data show that previously observed trends in laboratory strains mostly apply to these
conditions and strains. In general, the efficiency of amino acids to be used as nitrogen sources
and the production of major volatiles due to their presence followed the same patterns for both
industrial yeast strains. However, the production of the secondary metabolites butanol,
propanol, acetic acid, and ethyl acetate were found to be produced in different final
concentrations dependent upon the yeast strain.
The branched-chained and aromatic amino acids (BCAAs) treatments were observed to
have the most dramatic effects on major volatile production. Investigating the relationships
between the initial concentration of the BCAAs and the final concentrations of major volatile
compounds, it was found that the production of fusel alcohols and fusel acids due to the
degradation of BCAAs by S. cerevisiae could be predicted from the initial concentration of
BCAAs. While under simple nitrogen conditions the production of several other secondary metabolites such as butanol, propionic acid, valeric acid, decanoic acid and 2-phenylethyl
acetate were found to be correlated to the initial concentration of BCAAs in the media.
Future studies should focus on the validation of these trends in aroma production in real
grape musts under various fermentation temperatures for a number of industrial wine yeast
strains.